The present invention relates to a system and process for drying materials and more particularly relates to a drying system which uses a heated drying medium such as air to dry materials.
In a wide variety of industries, large amounts of materials must be dried before use or before undergoing further treatment such as incineration. These industries include, but are not limited to, the lumber industry, many food processing industries, the paper production industry, the waste treatment industry, and the fertilizer production industry. In such industries, the materials are typically dried by forcing relatively hot air over the materials to evaporate the moisture from the materials. With reference to FIG. 1, ambient air is drawn into an inlet 10 of a conventional system via a blower 12, which then forces the air over a heater 14 which heats the air to a temperature of, e.g., 170.degree. F. The heated air then travels over a plurality of superposed drying trays 16 which convey the material to be dried through the system in a direction which is usually opposite to the direction of flow of the air. After evaporating the moisture from the material, the heated and saturated air is discarded directly to the atmosphere via an outlet 18 of the system.
While this system adequately dries the materials, it is extremely inefficient since all of the energy used to heat the air is lost when the saturated air is discharged from the outlet of the dryer. In fact, in a typical industrial system in which ambient air at 70.degree. F. is heated to 170.degree. F. for drying, 1,000,000 BTU/hr must be consumed to dry the materials to the required moisture levels. In addition, the heated air also serves as a source of heat pollution when it is discharged into the atmosphere.
In order to increase the efficiencies of dryers, other types of dryers have been proposed which use a heat-exchanger which recoups some of the heat which would otherwise be lost when the air is discharged. Referring to FIG. 2, these systems typically employ a dryer having a regenerative heat exchanger 20 thermally connecting the inlet of the dryer to the outlet. Heat exchanger 20 removes some of the heat from the saturated air before the air is discharged from the dryer from an outlet 22, and transfers the extracted heat to air entering the dryer from an inlet 24. During this process, some of the moisture condenses out of the saturated air and is removed from the system via a drain 26. Meanwhile, the air which has been heated by the heat exchanger is drawn through a blower 27, is heated further by a heater 28, and then dries the material present in trays 29, thereby becoming saturated. The saturated air is then forced through heat exchanger 20 and subsequently discharged from the dryer in the manner discussed above.
This system is more efficient than earlier systems since at least some of the heat which would otherwise be wasted through the discharge of the saturated air is recouped. In the typical system in which air is drawn into the dryer at 70.degree. F. the air is heated to about 120.degree. F. by heat exchanger 20 so that only about 500,000 BTU/hr need be consumed by heater 30 to raise the temperature of the air to the desired temperature of 170.degree. F.
Despite the increased efficiencies, systems which recoup some of the wasted heat in this manner still exhibit several disadvantages.
First, the system is still relatively inefficient since the heat exchanger 20 is incapable of removing all of the heat from the discarded air or of transferring it to the incoming air. In fact, in the latter part of the drying stage when there is little evaporation and the air entering the heat exchanger 20 is thus no longer saturated, most of the heat added to the system is used to heat the air rather than to evaporate moisture and is thus discarded with the air.
Another disadvantage results from discharging the drying air out of the dryer. Many materials, such as sludge, hospital wastes, and refuse transfer particles to the air in the drying process which, at best, produce an unpleasant odor and, at worst, are contagious or toxic to the environment. In the types of systems discussed above, the air being discharged from the dryer must undergo a complicated and expensive scrubbing operation before being released to the atmosphere. However, even these operations often do not guarantee that all odoriferous, toxic, or contagious particles will be removed from the air before the air is discharged to the atmosphere.
Heated air is also used to dry clothes in industrial and residential clothes dryers. In the typical clothes dryer 30, illustrated in FIG. 3, a drum 34 is rotatably mounted in a housing 32 and is driven to rotate by a motor 36. A blower 38 draws air into an inlet 40 of housing 32 which typically opens into the interior of the building. The air is then heated from an ambient temperature of, e.g., 80.degree. F. to a suitable drying temperature of, e.g., 200.degree. F. by a heating element 42. The thus heated air is subsequently forced through the drum 34 in contact with the clothes 44 to be dried, where it receives moisture from the clothes and is cooled to a temperature of, e.g., 160.degree. F. The air is then drawn through a lint filter 46 before being discharged from an outlet 48 of housing 32. In order to avoid unnecessarily humidifying the air in the building, the outlet 48 typically connects to the exterior of the building. The heating element of the average residential dryer will expend about 5 to 6 Kw of energy during the drying process.
Even when the system described above operates at maximum efficiency at the beginning of a drying cycle when the clothes are relatively wet, only about 50% of the heat added to the air by heating element 48 is used to vaporize the water, while the remaining 50% is wasted when the moisture-laden air is discharged to the atmosphere. At the end of the drying cycle when the clothes are relatively dry, as little as 10% of the energy added to the air is actually used to vaporize water, while the remaining 90% is wasted. Thus, by failing to recirculate the air, the conventional dryer wastes a large percentage of the heat used to dry the clothes in the dryer. Moreover, since the air used for drying is drawn from the inside of the building and is thus often heated or air conditioned before being drawn into the dryer, the energy used to condition the air within the building is wasted when the air is discharged from the dryer to the exterior of the building. Additional energy must be expended to heat or air-condition the air lost from the building in the drying process.